custom lookup

Hi,

I created a custom lookup using X++code to filter a master data  ( !'*JIT*' ||  !'*SER*'). The code works fine and the drop down comes as coded.

But if I type a value (SER*) in the dialog box, then the system is dropping down the SER* values in the drop down which is against the custom lookup code.

How to disable this property?

Thanks,

Lakshmi